A commercially suitable Soft-X-Ray or EUV lithography facility will require an intense soft x-ray/EUV light source that can radiate within a specific wavelength region within the range of approximately 11 to 14 nm in the EUV part of the electromagnetic spectrum. Capillary discharge sources have been proposed that can be used in such a facility. One such embodiment of the proposed capillary discharge source was first described in U.S. Pat. No. 5,499,282 by William T. Silfvast issued on Mar. 12, 1996 and is incorporated herein by reference. That particular proposed source would operate in a lithium vapor electrically excited to within specific ranges of plasma electron temperatures (10-20 eV) and electron densities (1016 to 1021 cmxe2x88x923) which are required for optimally operating a lithium vapor discharge lamp at 13.5 nm. That same patent also proposed soft-x-ray lamps at wavelengths of 7.6, 4.86, and 3.38 nm in beryllium, boron, and carbon plasmas. These wavelengths, however, are not within the range of wavelengths required for EUV lithography. Although that patent described the general features of these lamps, it did not give the specific discharge current operating range that would minimize bore erosion and the emission of debris from the lithium lamp, or the appropriate range of bore sizes for operating such a lamp.
Another proposed discharge source for use with EUV lithography was the xe2x80x9cdifferentially pumped capillaryxe2x80x9d discharge source that was described in U.S. Ser. No. 09/001,696 filed on Dec. 31, 1997 entitled: Capillary Discharge Extreme Ultraviolet Lamp Source for EUV Microlithography and other Related Applications, by the same assignee, which is incorporated by reference.
Some problems have been observed with these discharge sources described above. During operation of the discharge the interior walls of the capillary erode causing debris to be emitted from the discharge source. Firstly, the debris can be destructive to the surrounding optics such as concave mirrors immediately adjacent to the capillary bore opening. The emitted debris can both form a layer on the mirror lowering its reflectivity and cause destructive pits on the concave surface of the mirror destroying its utility. Secondly, the resultant erosion also increases the diameter of the capillary bore size whereby the light imaging usefulness of the device is degraded.
The first object of the present invention is to provide a capillary discharge chamber having a useful bore diameter during the operation of a capillary discharge light source.
The second object of the present invention is to provide a capillary discharge chamber having a capillary bore that is adjusted for erosion to provide a constant bore diameter during operation of a capillary discharge lamp.
The third object of the present invention is to provide a capillary discharge chamber having a capillary bore that is adjusted radially to maintain a constant bore diameter during operation of a capillary discharge light source.
The fourth object of the present invention is to provide an emitting capillary discharge light source having an improved capillary discharge chamber for applications in EUV lithography, microscopy, materials processing, metrology and resist analysis.
A preferred embodiment of the invention is to provide an emitting capillary discharge source having a capillary discharge tube configuration in which the bore of said tube is constructed to maintain a constant bore diameter during operation of the source.
A second preferred embodiment is related to an emitting capillary discharge source, comprising an electrode means; an insulated capillary having an initial inner bore diameter, the capillary being adjacent to the electrode means; a gas for being inserted within the capillary; voltage means for causing a light emission from the capillary; and, means for maintaining the initial diameter of the initial bore diameter at a constant value over time, for additional light emissions.
A third preferred embodiment has said means for maintaining the initial diameter of the initial bore diameter at a constant value over time wherein the constant value is at least 110%, and optimally 105% of the initial inner bore diameter, for lithographic applications.
Further objects and advantages of this invention will be apparent from the following detailed descriptions of presently preferred embodiments which are illustrated in the accompanying drawings.